[HBR] HB-67

[hbr@mailman.qth.net]

In a message dated 10/1/2003 11:18:16 PM Eastern Daylight Time, waltah@earthlink.net writes:

> Jim made many excellent comments, beginning with:
> 
> > It would be interesting to know how good or bad the HB-67 really is.
> 
> Yep.  I said what I think, but measurements are a lot better than 
> theory.   

Years ago, I suggested that maybe ARRL should do "Product Reviews" on older gear that was made before the current lab was in operation. Wouldn't ya love to know how an SS-1R stacks up when tested in exactly the same manner as a new Ikensu?

They didn't think it was practical back then, but that was before TQN's old radio column, too.
> 
> > There does need to be enough gain to make up for the losses in the
> > input tuned circuit and mech filter, though. Also, having adequate
> > gain means no RF stage is needed. Finally, the front end has to be
> > gainy enough that the first rx stage and not the first if stage noise
> > is the dominant factor. 
> 
> All of this reduces to 'enough signal/input noise exiting the filter to 
> cover up the 1st IF stage noise.'   But on 75 meters this is not much 
> of a challenge because the atmospheric noise level on the band is so 
> high.   No reasonable mixer on this band requires any preceding gain 
> at all -- any triode mixer (for example) will work fine with no RF stage, 
> indeed, probably any decent pentode would deliver an adequate 
> noise performance.  

A lot of that depends on what you're using for an antenna. 

The Ancient Ones improved the classic "Four Tube Superhet" by replacing its 6SB7Y pentagrid mixer with a 6AC7. 

On higher bands the HB-67 adds a converter and 
> the properties of the converter then dominate noise considerations -- 
> but with a high gain second mixer (the 7360) you're looking at a great 
> deal of gain prior to the filter.  

I think a lot of that gain is lost due to impedance mismatching. One of the things I learned the hard way was that an easy way to build a zero-gain active mixer is to try to drive a crystal filter of with a few hundred ohms input Z from a tube without a matching network.
> 
> The highest performance homebrew receiver design I know of is the  
> G2DAF Mk II receiver in the RSGB *Handbook*, 4th edition.   The RF 
> stage is a push-pull 7ES8, AGC controlled and always operated at 
> less than unity gain.  The 1st mixer is a ECC88 (6DJ8), again, push-
> pull, driven by a push-pull crystal oscillator to convert to a tunable IF 
> 5-5.5 Mcs.  The 2nd mixer (still push-pull, an ECC85, 6AQ8) gets 
> single end input from a VFO, converting to a fixed IF of 455 kcs; the 
> selectivity is provided by a mechanical filter.   Claimed dynamic 
> range 100 db; claimed sensitivity <0.5 uV for 10 db s/n on all bands.  
> I know of no reason the claims should be doubted.   

oh mama

There is also the legendary CE 100R, of which exactly one is known to exist.

Any way to get a scan of the G2DAF receiver article? My RSGB Handbook is of early-'60s vintage. Has the G3PDM receiver with the locked-synthesizer LO. 
> 
> On 10 meters and above one can probably improve performance with 
> a low noise RF stage of modestly over unity gain.  Below there an RF 
> stage is at best an AGC-controlled attenuator and at worst a serious 
> contributor to mixer/filter overload.   Of course this assumes 1960s or 
> later tubes.   In the 1940s, the picture was different (6K8s for mixers 
> ...) and an RF stage would have been most helpful.   

Agreed.
> 
> The 1MHBR got an RF stage because I wanted to keep things 
> simple by distributing the gain over various frequencies and in a 
> design of this type I was willing to accept some degradation of the 
> dynamic range.  The HBR2K got an RF stage because I wanted 
> someplace to apply AGC ahead of the 1st mixer and because it 
> simplified using the existing front-end coils; the jury is still out on 
> whether >80 db dynamic range can be obtained.  
> 
The Southgate Type 7 got a 6EH7 RF stage because I wanted a controllable gainstage before the 7360 mixer, and because IF feedthrough was a problem.

> In a design 'from a blank sheet of paper' and aiming at top 
> performance, I'd follow G2DAF on the RF stage and 1st mixer.

Agreed. Is the G2DAF rx bandswitched?
> 
> > I always wondered why they did that, instead of another 7360. The 
> > 6BA6/6D10 converter did not save money, space or milliamps.
> 
> Considering the casualness of the original design, I think it was just 
> a mistake.  

With all due respect to the Handbook and its authors, a lot of the later receiver projects appear to be aimed more at ease of construction/duplication and simplicity than at getting the most out of a design. 
> 
> >> I believe that the later W6TC HBR designs will give better 
> >> performance.    
> 
> > All depends what you mean by "better". The HBR designs can be drifty
> > because of the high frequency LO. The selectivity in a W6TC rx is so
> > far from the antenna (3 stages until the first 100 kc. IF can, and
> > it's not the entire knothole) that dynamic range has to be
> > comrpomised compared to a single-conversion-one stage-before-
> > the-filter design.
> 
> These are valid criticisms. 

I'd call 'em tradeoffs. 

> But the HBR-series are not 'spare no 
> effort' designs -- they were intended to give *good* performance (i.e. 
> better than the average ham could have afforded in a commercial 
> receiver at the time) in a home brew set that was fairly simple to 
> duplicate and could be completed in steps, one band at a time.

I think that if you take a catalog of the HBR period and add up the price of all new parts to build one, the total reaches a surprising figure. Compared to, say, a Drake 2-B.

>   The 
> HB-67 is if anything a more casual design -- it is possessed of less 
> design integrity.  While it ought to have less warm up drift on the 
> higher bands (for equal care in construction) than the HBR-series, I'd 
> expect it to be a weaker performer in almost every other department.  
> Note that there's no buffer between the triode oscillator and the 
> mixer; a 7360 probably has less coupling between the beam and one 
> deflection plate than most mixers do between two grids but I wouldn't 
> assume that pulling will not be a problem.   And when you build the 
> necessary converter to get multi-band operation, it's not exactly 
> simple, either.   

My 1970s cereal-bowl rx on Kees' website had an LO buffer stage...
> 
> Regarding balanced drive to the beam tube mixer:
> 
> > Or better yet, use a phase splitter/isolator tube to get balanced
> > drive without all the complexity ... 
> 
> That's not exactly a way to simplify things. 

It beats the split-coil you need otherwise.

  A push-pull oscillator 
> inherently gives you two signals 180 degrees out of phase -- phase 
> balance is guaranteed by the physics of oscillation.  Thus you only 
> need to obtain amplitude balance -- a fairly simple operation.   If you 
> go to a phase splitter the cathode and plate signals won't be in 
> phase because they'll have different capacitive loads.  Okay, so you 
> fix that ... but the gain is different to the plate compared to the 
> cathode so the phase is *still* not quite right ... then you've got the 
> amplitude to deal with.   For narrow bands, you can probably work 
> out a two-adjustment scheme (per band) that will be satisfactory but 
> that's a bunch more work than just using a push-pull oscillator.

It'd be worth a try to find out, certainly.

The balanced beam deflection mixer is a real headache because you need to balance both the level and phase of both the LO drive and the output circuits - AND adjust the DC voltages on the electrodes to get equal gain on both plates. I gave up on it while building the Type 7.


> 
> As to the rest of your proposed design, I generally agree.  Certainly 
> premixing a tunable oscillator with crystals to get the desired band 
> and thus eliminating the second mixer on the signal path should 
> simplify the overload situation -- an amplifier is inherently more linear 
> than most practical vacuum tube mixers.   Just a few comments:
> 
> 1.  Premixing trades gives you more spurious response issues in 
> exchange for the reduced overload problems.   That's probably a 
> good trade-off for a ham design.  It also increases complexity 
> significantly because the premixer output must be tuned across each 
> band -- or broadbanded.   The added complexity is an issue in a 
> homebrew design.   

Been there! 
> 
> The trade-off really should be explored.   I don't know of a well-worked-
> out home brew design that uses premixing, 

The Type 7 has some spurs but one gets used to them. The stability and ability to transceive were judged to be more important. Simple bandpass couplers knock down the spurs a good deal.

> although one did appear 
> in QST by a W1 ...   

Where? I'd like to see that!

> The TR4 series are premixed, right?   They're 
> probably a good place to look.

Both the TR-3/TR-4 transceivers and the R-4x/T-4x pairs used premixing, but they are different schemes! Ten Tecs are premixed, too.
> 
> 2.  I need to see a Pullen mixer with circuit values and performance 
> measurements before I'll be convinced that the thing has much to 
> offer.   From my experience (which may possibly translate as "my 
> inept attempts to build one") it appears to be a low level switching 
> mixer -- that is, quite linear on the signal path for small signals.   If 
> so, its use is restricted to no-RF gain designs.   

I gotta build me one.
> 
> 3.  I'm not clear on the value of re-converting the IF before the final 
> detection.  Why shouldn't the BFO be on the IF?   With a high gain 
> IF you have to 'live clean' anyway -- shield and decouple most 
> carefully, so BFO pickup at the IF input isn't a problem; with a single 
> IF stage the answer is the same but easier to attain.

See the 1957 article "What's Wrong With Our Present Receivers?" and the "Miser's Dream" (1965). The problem of the BFO signal getting into the IF front end was there even in highly shielded/bypassed designs. 

The "rearend conversion" scheme permits such tricks as tuning the last conversion oscillar instead of the BFO, putting the detector on whatever freq you desire, and ease of keeping the BFO out of the AGC amp.
> 
> 4. Triode audio:  Nah ... use feedback.   And to make the best use of 
> that, you need bunches of extra gain -- use pentodes.   Start with the 
> RCA tube handbook hi fi amp designs.

Been there, done that, prefer my 12BH7 triode pushpull audio output stage. Simple as dirt and clean as a whistle. But you gotta have the audio iron.
> 
> 5.  The idea of using plug-in premixer subassemblies for the various 
> bands is interesting.   But I don't see any real problem with 
> bandswitching this, and ganging the tuning of the output with that of 
> the RF/Antenna coils -- it's just the regular superhet tuning problem 
> except that instead of tuning the vfo you're tuning the output of the 
> premixer.   Just need a variable cap, with a whole bunch of gangs 
> and a single coil/fixed cap selected with the crystal for each band, 
> since the higher bands (at least) will require overtone crystals. 
> Complexity ... it's an issue, but going plug in just gives 
> you a 
> different kind of complication.  Or so it seems to me.

Finding a suitable bandswitch is a real pain - been there, too. Using one constrains the mechanical design enormously, makes the metalwork more complex, and the switch shaft needs multiple ground points or it becomes a nice coupling device. (Another lesson). 

With plugins, you can switch many more points without the set becoming bigger than the desk. The circuits for each band don't need to be the same, either. The savings in metalwork alone may be worth it. 

The Southgate Type 5 transceiver (now dismantled, it was based on the Heathkit SB line heterodyne scheme because I found a CW filter at the 1978 Rochester hamfest for $5) used plug-in heterodyne oscillators and coils. Worked!

Here's the current concept for the Type 8, which is meant to be an 80/40/20 meter CW transceiver:

1) Dual-tuned input circuits (bandswitched)

2) Beam deflection or Pullen 1st mixer. Unit VFO of 5.1-5.4 MHz (with buffer) feeds mixer on all bands. 

3) 1st Mixer output is 8.9 MHz on 80 and 20, 12.4 on 40. Double tuned circuit with very high Q couples to 2nd mixer. Double tuned circuit is switched by means of small sealed Sigma relays

4) 2nd mixer converts 8.9/12.4 MHz IF to 1.4 MHz to feed the filters. VXO conversion osc is at 10.3/11 MHz (or 7.5/13.8). VXO permits RIT. Relay switches het osc

5) One or two stages of 6EH7 IF, with two 8 pole 500 Hz filters.

6) Product detector, xtal BFO, triode audio and sidetone generator. LC audio filter before final audio stage. 

7) Buffered output of VFO is fed to transmitting mixer. Het osc at 8.9/12.4 MHz results in transceive capability. (Transmitter on separate chassis

8) All het oscs have trimmers to permit exact setting for perfect transceive. Dial uses cap from BC-221 and six inch internally lighted Plexiglas drum. External power supplies. 

9) Controls: Main tuning, RIT, RIT on/off, RF gain, AF gain, sidetone gain, preselector, bandswitch, audio filter narrow/wide/out.

Comments?

73 de Jim, N2EY